1. Field of the Invention
The present invention relates to the fabrication of semiconductor devices. More particularly, the present invention relates to a photo mask used in a photolithography process in which a pattern is transcribed onto a semiconductor substrate.
2. Description of the Related Art
Generally, the fabrication of a semiconductor device includes several photolithography processes and etch processes to form circuit elements on a semiconductor substrate. Photolithography is a process in which the pattern of a photo mask is transferred to a layer of photoresist on the substrate. More specifically, the photolithography process involves coating the semiconductor substrate with photoresist, exposing the layer of photoresist to light directed through a photo mask so that the pattern of the photomask is transferred to the photoresist, and developing the exposed photoresist using a developer whereby the layer of photoresist is patterned. Subsequently, a layer underlying the photoresist pattern is etched using the photoresist pattern as an etch mask, and the photoresist pattern is removed.
Also, semiconductor devices have become much more highly integrated in recent years. Accordingly, the critical dimensions (CD) of semiconductor devices have become smaller and smaller. Thus, the critical dimensions of the patterns of the photo masks are becoming smaller, and the patterns themselves are becoming much finer. Under such circumstances, a mask error enhancement factor (MEEF) of the photolithography process has increased, and the process of fabricating an effective photo mask has become more difficult.
More specifically, in order to meet the current demand for a highly integrated semiconductor device, the photolithography process must produce a pattern on a substrate whose distribution of critical dimensions is beyond the capability of the photo mask. Furthermore, the complexity of the device requires that the pattern formed in a die of the substrate have a wide distribution of critical dimensions. This requirement makes it difficult to maintain shot uniformity on the semiconductor substrate. Thus, it is very difficult to fabricate an effective photo mask unless the photolithography process has a low MEEF. Recently, however, a method of compensating for the transmissivity of the photo mask has been employed to overcome these limitations. More specifically, the transmissivity of a predetermined portion of the photo mask is altered to produce a critical dimension different from that which is produced from the original photo mask pattern. A conventional method of compensating for the transmissivity of a photo mask will be explained with reference to FIGS. 1A through 1F.
Referring first to FIG. 1A, a photo mask 100 is prepared. The photo mask 100 includes a transparent substrate 105 such as a quartz substrate, a light-shielding layer pattern 110 on the transparent substrate 105, and a chrome pattern 115 disposed at the edge of the light-shielding layer pattern 110. The photo mask 100 may be a phase shift mask (PSM), and in this case, the light-shielding layer of the light-shielding layer pattern 110 is a phase shifter. Alternatively, the photo mask 100 may be a binary mask, in which case the mask consists of a transparent substrate and a light-shielding layer pattern.
A critical dimension of the photo mask 100 is the interval between the elements of the light-shielding layer pattern 110. The areas between these elements define transmission sites, respectively, at which the transparent substrate 105 is exposed and light is transmitted by the mask. That is, the critical dimension is the width of a transmission site. Each photo mask used in the fabrication of a semiconductor device is designed to have a predetermined target critical dimension. However, the actual critical dimension may differ from the target critical dimension due to fabrication errors, or the like. Furthermore, the pattern of a photo mask may have a plurality of critical dimensions, i.e., the critical dimension may vary across the photo mask.
The critical dimension of a semiconductor device is a very important factor in the performance of the device. In particular, the gate length in a highly-integrated device is on the sub-micron order and if this critical dimension of the gate does not correspond to the design CD, the device may experience serious operating defects caused by a short channel effect or the like. Similarly, if the size of a contact hole does not correspond to the design CD of the contact hole, the contact resistance or the like may be so high as to seriously impede the operating speed of the device. Therefore, fabricating semiconductor devices with precise critical dimensions becomes more important as the semiconductor devices become more highly integrated.
Referring to FIG. 1B, the back surface of the transparent substrate 105 of the photo mask 100 is coated with a layer of photoresist 120. Then, the photoresist 120 is exposed to light directed through another photo mask using an exposure apparatus such as a stepper (not shown). As shown in FIG. 1C, the photoresist reacts with the light, whereby its etch characteristics are changed; reference numeral 120′ in the figure designates an exposed portion of the photoresist 120.
Referring to FIG. 1D, the photoresist 120 is developed, thereby forming a photoresist pattern 120b. The developing operation may be a wet etch. In the case in which the photoresist 120 is a positive type of resist, the exposed portion 120′ of the photoresist is removed, as shown in drawing.
Referring to FIG. 1E, the back surface of the transparent substrate 105 is etched using the photoresist pattern 120b as an etch mask, thereby forming a transparent substrate 105a having a lattice structure whose pattern has a CD less than that of the wavelength of the exposure light (the light that will be used in the photolithography process). The etching operation may be a reactive ion dry etch.
Referring to FIG. 1F, the photoresist pattern 120b is removed, thereby completing the formation of the photo mask 100a. The transmissivity of the light passing through the transparent substrate 105 is altered by the lattice structure of the pattern at the back surface of the photo mask 100a. 
The transmissivity of the photo mask is thus affected by the diffraction of the light by the lattice structure (pattern) at the back surface of the transparent substrate. Therefore, the intensity and phase of the transmitted light can be controlled according to a duty ratio or fill factor of each portion of the lattice structure, and by the orientation of the pattern of the lattice structure. Furthermore, the transmissivity can be controlled across the photo mask by designing the lattice structure to have local characteristics, i.e. characteristics according to the position that it occupies in the photo mask. Furthermore, phase changes in the transmitted light may be realized just by providing a lattice of one binary phase. Accordingly, a multi-phase diffraction optical device may be realized by one process.
According to the conventional art as described above, variations in the distribution of the actual CDs from the design CDs of the photoresist pattern and of the ultimate semiconductor device formed using a photo mask may be compensated for by forming a lattice structure at the back surface of the photo mask to alter the transmissivity of the photo mask. However, the turn-around time (TAT) of fabricating the photo mask is slowed by the additional process of forming the lattice structure on the back surface of the photo mask. Furthermore, defects such as scratches or the like may occur on the front surface of the photo mask during the process of patterning the back surface of the photo mask.